1. Field of the Invention
The present invention relates to fuel gas burners that have very low NOx emissions and, in particular, to such burners that operate with flue gas recirculated into the combustion chamber.
2. Description of the Prior Art
There are three basic sources/mechanisms of NOx formation during the process of flame combustion. One is thermal NOx formed in the flame at high temperature by oxidation of atmospheric nitrogen (N2) present in combustion air or otherwise mixed with fuel. The amount of thermal NOx typically increases exponentially with the increase in peek flame temperature. Typical range of uncontrolled thermal NOx in boilers, or process heaters, is 60 to 400 ppm. A second is NOx formed from fuel bound nitrogen—FBN (this does not include atmospheric nitrogen). With small quantities of FBN this NOx is proportional to the FBN in fuel. A third is prompt NOx that is formed from atmospheric nitrogen via reactions with hydrocarbon radical present in the flame during oxidation. The residence time of this radical is relatively small and so is prompt NOx. Its typical range is from 2 ppm to 10-15 parts per million (ppm) and thus is important only when levels of NOx below 20-25 ppm are desired.
Many common fuels like natural gas, refinery gas and diesel oil have little or no fuel bound nitrogen. For these fuels, the main source of NOx is thermal NOx.
The main technique of controlling thermal NOx in boilers and heaters is by diluting the air fuel mixture with some substantially inert media (cooled combustion products—flue gas recirculation, steam, water injection, etc.) that absorbs some heat released during the combustion and lowers peak flame temperatures. In some combustion devices (premixed type combustion), increased excess air can be used instead of inert media to achieve the same dilution effect.
In most cases the inert media is recirculating flue gas, as it typically has minimal adverse effects on the process thermal efficiency and is most readily available. At the same time, however, recirculation of the flue gas substantially increases the energy required for passing the mixture flow of combustion air and added flue gas through the system. An addition of 10% of flue gas recirculation (FGR) from the existing boiler exhaust back to the burner, for example, typically results in a 40-45% increase in the required power of the fan. This is especially critical in retrofits of large boilers with high-pressure losses through its convection passes.
There are some advanced burners that utilize the pressure energy of fuel to promote internal circulation of the flue gas inside the boiler, or heater radiant section. The effectiveness of these devices depends strongly on the temperature of gas surrounding the flame body. In large boilers, the furnace gas surrounding the flame has a temperature not much lower than the peek flame temperature. Thus, its effect on reducing the flame temperature and thermal NOx is greatly diminished. Recirculation of the gas within the confines of the radiant section back into the flame body does not increase the flow through the convection section of the boiler and through the fan. Thus, it does not impact directly the required fan horsepower.
There are also devices that use energy from high velocity combustion air jets to promote recirculation within the radiant section. The effectiveness of these techniques depends on the available burner pressure drop and temperature of furnace gas that is being mixed with combustion air. It is usually justified in boilers with low heat release. In boilers with high space heat release, the cross section of the furnace (boiler radiant section) is comparable to the flame cross section. In such boilers the temperature of gas surrounding the flame at the furnace front is substantially higher than in more liberal boilers. So the effectiveness of this gas for the purpose of flame temperature reduction and NOx control is diminished. The average velocity of combustion products through the furnace is also higher in high space release boilers. This makes it more difficult to return a substantial portion of combustion products back to the burner.
In addition to the factor of increased operating costs of burners with high amounts of FGR, another problem in achieving very low NOx levels, typically below 10 ppm, is maintaining a stable flame without strong oscillations. Overcoming this problem has typically required more expensive combustion controls with improved accuracy.